- what will it be used for?
- does she want a large keyboard?
- what OS has she got used to already? (Win 7 is different from XP, for example)
- how much does portability matter?

The last one's pretty important if her eye sight's not so good. If she'd prefer a big screen at a desk at all times then you could look at just getting her a ChromeBox, which is like a sort of Mac Mini but it's Chrome OS so that also means no more moving parts, saving things, etc.